Furnaces that process refinery feedstocks, particularly feedstocks high in sulfur compounds, are subject to fouling at temperatures of xcx9c700xc2x0 F. Typically the foulant consists of both inorganic corrosion products and carbonaceous deposits. Fouling adversely affects process economics by shortening furnace run lengths. While a conventional pigging process is effective in cleaning the furnace tubes, such cleaning exposes fresh tube metal to corrosive attack by sulfur compounds and in turn accelerated fouling. What is needed is an effective cleaning method that is capable of protecting the unit from corrosive attack by sulfur containing compounds and hence prevents fouling.
The invention includes a two step cleaning method for metal surfaces, which protects the surfaces from fouling. The method is particularly applicable to units which process sulfur containing feeds in which fouling occurs due to metal surface corrosion caused by the sulfur containing compounds in the feeds being processed in the units.
A method for cleaning the surface of an alloy said alloy comprising a base metal and an alloying metal, wherein said alloying metals are selected from the group consisting of chromium, chromium in combination with silicon, chromium in combination with aluminum and chromium in combination with silicon and aluminum, wherein said base metal of said alloy is selected from iron, nickel, cobalt and mixtures thereof, comprising the steps of:
(a) pigging said alloy surface; and thereafter
(b) passivating said alloy surface by contacting said surface with a gas comprising steam for a time and at a temperature sufficient to form at least one mixed oxide layer on said alloy wherein said mixed metal oxide contains an average alloying metal content of from equal to the alloying metal content in said alloy up to 100% alloying metal.
A method for increasing the run length in a refinery process conducted in a unit having alloy surfaces susceptible to fouling, said alloy comprising a base metal and an alloying metal, wherein said alloying metals are selected from the group consisting of chromium, chromium in combination with silicon, chromium in combination with aluminum and chromium in combination with silicon and aluminum, wherein said base metal of said alloy is selected from iron, nickel, cobalt and mixtures thereof, comprising the steps of:
(a) pigging said alloy surface; and thereafter
(b) passivating said alloy surface by contacting said surface with a gas comprising steam for a time and at a temperature sufficient to form at least one mixed oxide layer on said alloy surface wherein said mixed metal oxide contains an average alloying metal content of from equal to the alloying metal content in said alloy up to 100% alloying metal.
Pigging is a well-known method of cleaning metal surfaces in process/transportation pipelines. For example, the skilled artisan need only refer to xe2x80x9cRecent Innovations in Pigging Technology for the Removal of Hard Scale from Geothermal Pipelines,xe2x80x9d Arata, Ed; Erich, Richard; and Paradis, Ray, Transactions-Geothermal Resources Council (1996), 20, 723-727, Mitigation of Fouling in Bitumen Furnaces by Pigging, Richard Parker and Richard McFarlane, Energy and Fuels 2000, 14, 11-13, or other known references.